Voltage clamp

ABSTRACT

A voltage clamp circuit which operates using a voltage controlled current source where the change of the polarity of the voltage controlled current source controls whether it is clamping or not. While clamping, the stability of the control loop uses the capacitance of the output to create and single pole roll-off of the loop gain and while not clamping, uses the capacitance of the circuit which sets the clamping voltage to produce the roll-off. The circuit operates in a linear fashion both while clamping and not clamping, which allows for a faster response when clamping is needed.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication No. 62/082,841, titled VOLTAGE CLAMP and filed on Nov. 21,2014.

TECHNICAL FIELD

This disclosure relates generally to voltage clamps and, moreparticularly, to voltage clamps designed for dry circuit ohms clamping.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a loop concept circuit on which certainvoltage clamp design embodiments in accordance with the disclosedtechnology are based.

FIG. 2 is a block diagram illustrating an example of a voltage clampcircuit coupled with a device under test (DUT) in accordance withcertain embodiments of the disclosed technology.

FIG. 3 is a block diagram illustrating an alternative example of avoltage clamp circuit coupled with a DUT in accordance with certainembodiments of the disclosed technology.

DETAILED DESCRIPTION

Dry circuit resistance generally requires a sourcing circuit that limitsthe maximum applied voltage across the device under test (DUT) to lessthan a few tens of millivolts (e.g., 20 mV to 30 mV). This is typicallyneeded to prevent the breaking down of a thin oxide that can form onelectrical contacts within the DUT, such as a relay contact or a contactwithin a connector pair.

Embodiments of the disclosed technology are generally directed to acircuit having a maximum voltage limit and a maximum current limit. Thecircuit may include a first current source configured to drive theoutput of the circuit (i.e., by setting the maximum current) and asecond current source having a polarity that is opposite the polarity ofthe first current source, and is coupled with a resistance.

A voltage-controlled current source may be configured to, when itscurrent has a polarity opposite that of the first current source, removecurrent from the output and, when its current has a polarity that isopposite that of the second current source, remove current from the nodecommon to the second current source and the resistance, thus reducingthe current through the resistance;

The circuit may include a component configured to compare the outputvoltage to the voltage across the resistance and drive the voltagecontrolled current source's input such than there is a negativefeedback.

The circuit may include a first capacitance added to the output of thecircuit to provide a single pole roll off of the output voltage from theinput to the voltage controlled current source. The circuit may alsoinclude a second capacitance added across the resistance to provide asingle pole roll off of the voltage across the resistance from the inputto the voltage controlled current source.

FIG. 1 illustrates an example of a loop concept circuit 100 on whichcertain voltage clamp design embodiments in accordance with thedisclosed technology are based. In the example 100, a capacitor C causesa 20 dB/dec roll-off where g_(m)/c represents the gain-bandwidth of thecircuit. The current source g_(m) should have a constant gain forfrequencies less than or equal to g_(m)/c.

FIG. 2 is a block diagram illustrating an example of a voltage clampcircuit 200 coupled with a DUT in accordance with certain embodiments ofthe disclosed technology. In the example, a current source I₀ is steeredby the two diodes D₁ and D₂ to regulate the voltage on one of twocapacitors C₁ or C₂. When the I₀ current is negative, the Ohm's currentsource I₁ is diverted through the first diode D₁, thus reducing thecurrent flowing to the HI connection.

When the I₀ current is positive, however, the Is current source issupplied through the second diode D₂, thus reducing the current flowingthrough a source resistor R_(s). The loop circuit 200 generally eitherregulates V₂/R₂ to be equal to −V₂/R₂ (i.e., with the first diode D₁conducting) or regulates V₂/R₂ to be equal to −V₂/R₂ (i.e., with thesecond diode D₂ conducting).

In the example 200, a voltage V₁ may be clamped to R₂/R₂ R_(S)I_(S).Thus, R₂/R₂ R_(S)I_(S) generally needs to be set between 20 mV and 30 mVto meet the needs for dry circuit testing. The loop bandwidth isg_(m)/C₂ when D₁ is conducting and switches to g_(m)/C₂ when D₂ isconducting.

The ohm's current source protection will generally drop voltage whencurrent is flowing through it. Therefore, the capacitor C₂ and sourceresistor R_(s) may be bootstrapped to follow the ohm's current source'svoltage. This advantageously prevents both of the diodes D₁ and D₂ fromturning on at the same time. A differential stage, Diff, may be used totranslate the voltage across the source resistor R_(s) to ground to bemixed against the voltage V₁.

In the example 200, the first capacitor C₁ is used to limit the ratethat the voltage V₁ rises when a conduction path (e.g., the DUT) betweenthe HI and LO connections is suddenly removed, e.g., to minimize theovershoot. Using the capacitance-g_(m) interaction to control stabilitygenerally avoids problems that the capacitance may present in situationswhere a normal voltage source loop is used.

FIG. 3 is a block diagram illustrating an alternative example of avoltage clamp circuit 300 coupled with a DUT in accordance with certainembodiments of the disclosed technology. In the example 300, the clampvoltage for V₁ is I_(s)*R_(s).

Having described and illustrated the principles of the invention withreference to illustrated embodiments, it will be recognized that theillustrated embodiments may be modified in arrangement and detailwithout departing from such principles, and may be combined in anydesired manner. And although the foregoing discussion has focused onparticular embodiments, other configurations are contemplated.

In particular, even though expressions such as “according to anembodiment of the invention” or the like are used herein, these phrasesare meant to generally reference embodiment possibilities, and are notintended to limit the invention to particular embodiment configurations.As used herein, these terms may reference the same or differentembodiments that are combinable into other embodiments.

We claim:
 1. A voltage clamp circuit, comprising: a first current sourceconfigured to drive an output of the circuit, the first current sourcehaving a polarity; a second current source coupled with a resistance,the second current source having a polarity that is opposite thepolarity of the first current source; a voltage-controlled currentsource having a polarity and configured to: when its polarity isopposite that of the first current source, remove current from theoutput; and when its polarity opposite that of the second currentsource, remove current from a node common to the second current sourceand the resistance; a first capacitance coupled with the output; and asecond capacitance coupled across the resistance.
 2. The circuit ofclaim 1, further comprising a component configured to compare the outputvoltage to the voltage across the resistance.
 3. The circuit of claim 2,wherein the component is further configured to drive an input of thevoltage-controlled current source such than there is a negativefeedback.
 4. The circuit of claim 1, wherein the first current source isconfigured to drive the output of the circuit by setting a maximumcurrent.
 5. The circuit of claim 1, wherein the first capacitance isconfigured to provide a single pole roll off of the output voltage fromthe input to the voltage-controlled current source.
 6. The circuit ofclaim 1, wherein the second capacitance is configured to provide asingle pole roll off of the voltage across the resistance from input tothe voltage controlled current source.